fix crash which could happen when closing an invalid socket

for some reason boost::asio doesn't account for this and lets us simply
die when/if this happens.

include/Client.h

@@ -127,6 +127,7 @@ private:
     std::string mDID;
     int mID = -1;
     std::chrono::time_point<TimeType> mLastPingTime;
+    std::mutex mDisconnectMtx;
 };
 
 // Returns a valid client, or nullptr if no such client exists

src/Client.cpp

@@ -59,7 +59,13 @@ std::string TClient::GetCarPositionRaw(int Ident) {
 }
 
 void TClient::Disconnect(std::string_view Reason) {
+    // we need exclusivity here in case we can't
+    std::unique_lock Lock(mDisconnectMtx);
     beammp_debugf("Disconnecting client {} for reason: {}", GetID(), Reason);
+    if (mSocket.is_open()) {
+        beammp_debug("Somehow Disconnect() was called twice, ignoring");
+        return;
+    }
     boost::system::error_code ec;
     mSocket.shutdown(socket_base::shutdown_both, ec);
     if (ec) {